1. Field of the Invention
The present invention relates to a high-strength heat insulating material containing no asbestos and to a method of producing such a heat insulating material.
2. Description of the Art
An electrically insulating cement plate (commonly called hemit or lumber) defined in Japanese Industrial Standard (JIS) C2210 is formed by molding asbestos using Portland cement as a binder. Since the hemit is easily cut and has an excellent heat-resistant property and high mechanical strength with good machining characteristics such as surface cutting property, boring property and the like, it is used not only as an electrical insulator but also as a heat insulator. For example, the material may be used to fabricate a heat insulating plate for a heated press, a casing for an induction furnace, or the like, which must be cut before use and must be of high strength.
However, since the above JIS standardized goods contain 40% asbestos, a large quantity of harmful asbestos fiber dust is produced. Therefore, there is a need to provide a non-asbestos material having similar strength and workability properties to the asbestos-containing material.
Conventionally, a heat insulating material having a density of 1.6-2.0 g/cm.sup.3 obtained by binding talc powder and pulp using Portland cement (Japanese Patent Unexamined Publication No. Sho-61-109205) has been proposed as a cement-quality substitute containing no asbestos. However, the proposed material, has a disadvantage in that its mechanical strength and size are significantly reduced due to heating, because the only reinforcing fiber is pulp. Further, its toughness is insufficient. When the material is heated under application of a large load or under a constrained condition, The material is apt to crack. Therefore, the material has not gained acceptance as a substitute for hemit or the like, even though the material does not contain any asbestos.